An electric machine such as an electrical power generation system, genset, or the like, is generally used to convert one form of energy into another and may operate in a motoring mode to convert electrical power into mechanical or rotational power, or operate in a generating mode to convert mechanical or rotational power into electrical power. Among the various types of machines available for use with an electric drive assembly, switched reluctance (SR) machines have received great interest for being robust and cost-effective. While currently existing systems and methods for controlling such electric machines provide adequate control, there is still room for improvement.
In order to efficiently operate the electric drive of a machine, it is important to accurately monitor the speed and/or position of the rotor relative to the stator of the machine associated with the electric drive, not only at high speeds of operation but at zero or low speeds as well. Conventional electric drives which operate at such lower speeds of operation or often change its rotational direction thus detect the absolute position of the rotor by using a high resolution speed wheel that is coupled thereto and provided with a plurality of teeth that are detectable by some form of a proximity sensor. Based on the desired use or application, however, not all machines may be able to use such high resolution speed wheels, but rather, may be required to use a low resolution speed wheel with much fewer teeth provided thereabout.
Use of such low resolution speed wheels makes it difficult for the electric drive system to accurately determine the absolute rotor speed or position and produce sufficient torque, especially while operating at zero or low speeds, such as during engine start routines. In some such designs, an initial rotor position is calculated at zero or close to zero speeds, from which further calculations and sensor feedback are relied upon to track absolute rotor position during operation of the machine. However, due to the low resolution of the speed wheels being used and limitations associated with the sensors, the initial rotor position calculations may be inaccurate. As the integrity of the tracking and overall control of the machine relies on the accuracy of the initial rotor position, any offsets caused by an inaccurate initial rotor position reading may be left uncorrected, which can further cause poor machine performance or prevent operation of the machine altogether.
Accordingly, there is a need to provide simple and more reliable means to control the operating speed of electric machines having lower resolution speed wheels. The disclosed systems and methods are directed at addressing one or more of the needs set forth above and overcoming the deficiencies of the prior art.